Transition-metal-containing crystalline compounds of spinel structure are important constituents of inorganic coatings employed for an extremely wide variety of applications. For example, iron-containing spinels such as magnetite (Fe.sub.3 O.sub.4), and also other ferrites such as manganese, cobalt and nickel ferrites, have been applied to a variety of organic and inorganic substrates in order to impart desirable electromagnetic properties thereto. Processes for applying such coatings and controlling the properties thereof in the course of deposition comprise a large body of technology and include vapor deposition, precipitation, evaporation and thermal decomposition techniques.
Similarly, a large number of transition metal spinels are included within the classes of catalytically-active compounds, and have been applied in the form of coatings to a variety of metal, glass and ceramic supports for use in catalytic processes. Among the transition metal spinels useful in catalysis are Mn.sub.3 O.sub.4, MnAl.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, CuAl.sub.2 O.sub.4 and CuCr.sub.2 O.sub.4.
In virtually all applications wherein transition metal spinel coatings are employed in combination with inorganic substrates, coating adherence and durability are extremely important properties. In many applications, binders are required to enable these coatings to demonstrate the toughness needed to withstand the shocks and abrasion of use. In the case of inorganic substrates exhibiting low surface porosity, bonding agents or high sintering temperatures are required to obtain coatings demonstrating a useful degree of adherence to the support. In the case of magnetic recording media such as magnetic memory discs, extreme surface flatness is required, compounding the difficulty of providing a durable, adherent coating.
In catalytic applications, particularly where high temperatures are involved, interactions between active spinel coatings and incompatible binders and/or supporting materials can cause reductions in activity due to the growth of inactive reaction phases or sintering. Yet the attainment of good adherence to commercial supports often requires the use of binders and/or high-temperature sintering treatments.
What is therefore desired is a means for providing transition-metal-containing spinel surface layers on compatible inorganic supports wherein bonding agents, binders, and high sintering temperatures are not required to obtain adherence, toughness and stability in the spinel film.